Genetics question

[virtualmaster999]

Hey everyone!

I'm kinda stuck on this problem.

"A color blind man without hemophilia (both X-linked traits) marries a woman who is a carrier for both traits. What is the probability they will have a son with both color blindness and hemophilia? Hemophilia and color blindness are unlinked genes"

I got 1/16 (1/4 prob of having a son with colorblindness x 1/4 prob of having a son with hemophilia)...but the answer is 1/8!

Please help me out with this. Thanks!!

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[FeralisExtremum]

This question is a bit tricky, I assume due to phrasing...I'll give it my best shot.

So, the man is color blind without hemophilia. This means he will give either a Y chromosome to a child, or an X chromosome carrying color blindness (but not hemophilia). Ultimately we know he must be passing on his Y chromosome since the question asks about a son. The mother is a carrier for both traits - so there are two possibilities for her chromosomes:

1. One X-chromosome with the genes for hemophilia AND colorblindness, and one X-chromosome without any issues
2. One X-chromosome with the gene for hemophilia (but normal for color blindness), and one X-chromosome with the gene for color blindness (but normal for hemophilia).

#1 has to be the case - it is the only way to have a son with both conditions simultaneously. Since the genes are unlinked, I think the implication is that the chance of #1 being the case is 1/2 (since 1. and 2. are equally likely possibilities).

Chance of having a son: 1/2
Chance of mother's X chromosome carrying both hemophilia AND colorblindness (see explanation above): 1/2
Chance of inheriting mother's affected X chromosome: 1/2

1/2 * 1/2 * 1/2 = 1/8 chance of having a son with color blindness and hemophilia. Remember, the question asks about the probability of having a son with both (simultaneously), not either/or.

 

[virtualmaster999]

This question is a bit tricky, I assume due to phrasing...I'll give it my best shot.

So, the man is color blind without hemophilia. This means he will give either a Y chromosome to a child, or an X chromosome carrying color blindness (but not hemophilia). Ultimately we know he must be passing on his Y chromosome since the question asks about a son. The mother is a carrier for both traits - so there are two possibilities:

1. One X-chromosome with the genes for hemophilia AND colorblindness, and one X-chromosome without any issues
2. One X-chromosome with the gene for hemophilia (but normal for color blindness), and one X-chromosome with the gene for color blindness (but normal for hemophilia).

#1 has to be the case - it is the only way to have a son with both conditions simultaneously. Since the genes are unlinked, I think the implication is that the chance of #1 being the case is 1/2 (since 1. and 2. are equally likely possibilities).

Chance of having a son: 1/2
Chance of mother's X chromosome carrying both hemophilia AND colorblindness (see explanation above): 1/2
Chance of inheriting mother's affected X chromosome: 1/2

1/2 * 1/2 * 1/2 = 1/8 chance of having a son with color blindness and hemophilia. Remember, the question asks about the probability of having a son with both (simultaneously), not either/or.

So unless the question specifically states "of all children" or says "prob of a boy/girl having the disorder", you have to multiply by 1/2 for the probability of male or female?

 

[virtualmaster999]

This question is a bit tricky, I assume due to phrasing...I'll give it my best shot.

So, the man is color blind without hemophilia. This means he will give either a Y chromosome to a child, or an X chromosome carrying color blindness (but not hemophilia). Ultimately we know he must be passing on his Y chromosome since the question asks about a son. The mother is a carrier for both traits - so there are two possibilities:

1. One X-chromosome with the genes for hemophilia AND colorblindness, and one X-chromosome without any issues
2. One X-chromosome with the gene for hemophilia (but normal for color blindness), and one X-chromosome with the gene for color blindness (but normal for hemophilia).

#1 has to be the case - it is the only way to have a son with both conditions simultaneously. Since the genes are unlinked, I think the implication is that the chance of #1 being the case is 1/2 (since 1. and 2. are equally likely possibilities).

Chance of having a son: 1/2
Chance of mother's X chromosome carrying both hemophilia AND colorblindness (see explanation above): 1/2
Chance of inheriting mother's affected X chromosome: 1/2

1/2 * 1/2 * 1/2 = 1/8 chance of having a son with color blindness and hemophilia. Remember, the question asks about the probability of having a son with both (simultaneously), not either/or.

For example, if the question asked "the probability that their son will have both disorders" then you would do 1/2*1/2=1/4 right?

 

[FeralisExtremum]

Depends entirely on the phrasing. If they ask what the probability of having a son with (insert random condition) is, then you have to account for the chance of having a son * the chance of having the condition. If they say something like "If the mother and the father have a son, what is the probability he has (insert condition)", then you don't need to account for the chance of having a son because it's already assumed in the question.

 

[virtualmaster999]

Depends entirely on the phrasing. If they ask what the probability of having a son with (insert random condition) is, then you have to account for the chance of having a son * the chance of having the condition. If they say something like "If the mother and the father have a son, what is the probability he has (insert condition)", then you don't need to account for the chance of having a son because it's already assumed in the question.

Perfect, and great explanation. Thank you very much Feralis!

 

[AK28]

This question is a bit tricky, I assume due to phrasing...I'll give it my best shot.

So, the man is color blind without hemophilia. This means he will give either a Y chromosome to a child, or an X chromosome carrying color blindness (but not hemophilia). Ultimately we know he must be passing on his Y chromosome since the question asks about a son. The mother is a carrier for both traits - so there are two possibilities for her chromosomes:

1. One X-chromosome with the genes for hemophilia AND colorblindness, and one X-chromosome without any issues
2. One X-chromosome with the gene for hemophilia (but normal for color blindness), and one X-chromosome with the gene for color blindness (but normal for hemophilia).

#1 has to be the case - it is the only way to have a son with both conditions simultaneously. Since the genes are unlinked, I think the implication is that the chance of #1 being the case is 1/2 (since 1. and 2. are equally likely possibilities).

Chance of having a son: 1/2
Chance of mother's X chromosome carrying both hemophilia AND colorblindness (see explanation above): 1/2
Chance of inheriting mother's affected X chromosome: 1/2

1/2 * 1/2 * 1/2 = 1/8 chance of having a son with color blindness and hemophilia. Remember, the question asks about the probability of having a son with both (simultaneously), not either/or.

But wouldn't the chance of having a boy with both of them be 1/4 because out of the two possibilities, the boy could get either the X chromosome with both conditions, no condition, just hemophilia, or just colorblindness?